The present invention relates to a master cylinder and, more particularly, to a master cylinder suitable for use in a brake-by-wire type brake system (hereinafter referred to as "BBW system") in which a brake pedal operating quantity (stroke, pedal force, etc.) is detected, and braking force corresponding to the detected brake pedal operating quantity is generated by supplying a high-pressure brake fluid from an external fluid pressure source to a wheel cylinder under control.
In general, this type of BBW system is equipped with a failsafe master cylinder that is connected to the brake pedal to supply a high-pressure brake fluid to the wheel cylinder instead of the external fluid pressure source in case of a system failure. The BBW system is also equipped with a stroke simulator, which operates as follows. When the system capable of supplying the brake fluid to the wheel cylinder from the external fluid pressure source under control is in a normal operative state, the stroke simulator provides pedal response, i.e. appropriate resistance to the depression of the brake pedal, to make the driver feel the brake system responding to the braking operation. There are two types of stroke simulators used in such BBW systems: a fluid pressure absorbing type in which an accumulator for containing the pressurized hydraulic fluid from the master cylinder is connected to the output side of the master cylinder to absorb the fluid pressure delivered from the master cylinder; and a pedal stroke type in which the brake pedal and the master cylinder are mechanically connected through a resilient member so that when the system is in a normal state, only the pedal stroke of the brake pedal is effected.
The fluid pressure absorbing type suffers, however, from the problem that when the master cylinder starts a stroke, the starting resistance of the piston seal in the accumulator is added to the resistance to the depression of the brake pedal. Therefore, the stroke simulator requires the driver to exert a pedal input corresponding to the sum total of the starting fluid pressure of the master cylinder itself and the starting fluid pressure of the accumulator. Consequently, the brake pedal feel becomes heavy.
The pedal stroke type unfavorably needs to change the structure around the brake pedal because of the need for providing a resilient member between the brake pedal and the master cylinder.
To solve the above problems, U.S. Pat. Nos. 5,720,170 and 5,729,979 disclose inventions in which the primary piston in the master cylinder is split to make the axial length of the primary supply chamber variable, thereby imparting the function of a stroke simulator to the primary piston. However, the structures of the two inventions have the problem that the cylinder bore is a stepped bore (with three stepped portions) and hence difficult to form by machining. In addition, the supply chamber portion of the primary piston, which exhibits the simulator function, is not in the course of flow of the hydraulic fluid from the reservoir to the discharge opening of the master cylinder, but present in the form of a blind alley with respect to the flow. Accordingly, it is difficult to carry out the hydraulic fluid charging and bleeding operation smoothly.